This application claims priority from Korean Patent Application No. 10-2004-0076902, filed on Sep. 24, 2004, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to an illumination unit and an image projecting apparatus and, more particularly, to an illumination unit using a light emitting diode (LED) and an image projecting apparatus employing the same.
2. Description of the Related Art
In general, image projecting apparatuses include an illumination unit to illuminate an optical modulation element. A metal halide lamp or a super high pressure mercury lamp are used as a light source of the illumination unit. Since the metal halide lamp and the super high pressure mercury lamp are very large, the illumination unit becomes large accordingly. Also, since the life span of these lamps are several thousands hours at the best, the lamps for home use need to be replaced frequently. To solve the above problems, a study to use a compact light source such as an LED module having a relatively long life span as a light source has been performed.
The LED module generally has a smaller amount of light than the metal halide lamp or the super high pressure mercury lamp. Thus, an LED module array formed by arraying a plurality of LED modules is used as a light source of the image projecting apparatus. A lens is used to condense light, in which an optical efficiency is deteriorated.
Referring to FIGS. 1 and 2, the deterioration of the optical efficiency will be described in detail. The multiplication of the size and angle of an image at a near axis area is preserved. Thus, the multiplication of a light emitting area of the LED module and a solid angle of a light emitting angle becomes the amount of preservation, which is referred to as etendue. As shown in FIG. 1, when a single LED module is to be used, the multiplication of the light emitting area ΦL and the solid angle UL of the LED module can be identical to the multiplication of a light emitting area ΦP and the solid angle UP of the optical modulation element.
When a plurality of LED modules are used in an array, as shown in FIG. 2, a light emitting area ΣΦL of the LED module array becomes larger than the light emitting area ΦL when a single LED module is used. The light emitting angles UL of the LED module and the LED module array are identical and the areas ΦP of the optical modulation elements therebetween are identical as well. Thus, to preserve etendue, when the LED module array is used, the solid angle UP′ of the light emitting angle of the optical modulation element increases compared to a case of using a single LED module. Since the angle effectively projected by a projection lens is UP, light in a range of an angle greater than UP cannot be effectively projected by the projection lens. Thus, loss as shown in FIG. 2 is generated to deteriorate the optical efficiency. As a result, an increase in the brightness of the image projecting apparatus is limited although the number of LED modules increases.